Internal combustion engines may include evaporative fuel recovery systems that have carbon fuel vapor canisters coupled to a fuel tank for absorbing fuel vapors. The canisters are also coupled to an engine intake manifold through an electronically controlled canister purge valve (CPV). Under purge conditions, fuel vapors vented from the fuel tank and captured in the canisters are drawn into the engine, where the vapors are combusted along with fuel injected by fuel injectors. A flow rate of the fuel vapors may be controlled via the CPV. The CPV may be a pulse width modulated solenoid valve that is actuated by pulse width modulated signals that are ON for a fraction of a period of the pulse and OFF for the remainder of the period. The CPV may open to allow fuel vapors to enter the engine during the ON state and may close during the OFF state.
One approach for operating the solenoid valve includes generating the pulse-width-modulated signal by utilizing a voltage supplied by a vehicle battery, and applying the signal to open the solenoid valve. However, the inventors herein have identified issues with such an approach. As an example, a battery state of charge may vary from a fully charged state to a discharged state during vehicle operation. Consequently, fuel vapor flow rate may vary. In particular, during low flow conditions, when the intake manifold vacuum is below a threshold, a high voltage input may result in higher purge flow rates than desired, whereas a low voltage input may result in insufficient purge. Consequently, due to large variation in the battery state of charge, control of purge valve during low intake vacuum conditions may be reduced.
Further, there may be delay in adjusting the valve from a closed state to an open state (herein referred to as opening response time) and/or in adjusting the valve from the open state to the closed state (herein referred to as closing response time). For example, if the opening response time is greater than the closing response time, the purge flow rate may be less than desired, and if the opening response time is less than the closing response time, purge flow rate may be greater than desired. Due to variations in the purge flow rate resulting from variations in the battery state of charge, and the delayed solenoid valve response times, an engine air-to-fuel ratio control may be reduced leading to reduced fuel economy and/or increased emissions.
In one example, the above issues may be at least partly addressed by a method for an engine comprising: during fuel vapor purging, applying a signal to an electronically controllable solenoid valve coupling a fuel vapor canister and an intake manifold of the engine in synchronization with a crankshaft position; wherein, a pulse width of the signal is based on an offset duration determined based on an instantaneous system voltage, an opening response time of the solenoid valve and a closing response time of the solenoid valve.
As an example, when purging conditions are met, a pulse-width modulated signal may be applied to the solenoid valve to open the solenoid for a desired duration to deliver a desired volume of fuel vapors. A pulse width of the signal (that is, a duration of solenoid open state) may be compensated based an offset duration. The offset duration may be determined based on a system voltage to compensate for variation in the system voltage. Further, in order to reduce variations in the purge flow rate due to the solenoid valve response times, the offset duration may be further adjusted based on the opening response time and the closing response time of the valve. Still further, purging of fuel vapors may be synchronized with a cylinder event (e.g. an intake stroke) in order to improve cylinder-to-cylinder distribution of fuel vapors and reduce fueling noise. For example the waveform may have a base frequency equal to cylinder firing frequency of the engine cylinders of the engine.
In this way, by delivering the signal with a pulse-width compensated for voltage variations and valve response times, improved purge flow control in a wide-voltage range may be achieved. Further, applying the signal in synchronization with engine operation may result in improved fuel vapor distribution.
The above advantages and other advantages, and features of the present description will be readily apparent from the following Detailed Description when taken alone or in connection with the accompanying drawings.
It should be understood that the summary above is provided to introduce in simplified form a selection of concepts that are further described in the detailed description. It is not meant to identify key or essential features of the claimed subject matter, the scope of which is defined uniquely by the claims that follow the detailed description. Furthermore, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure.